1. Field of the Invention
Generally, the subject matter disclosed herein relates to the manufacturing of integrated circuits, and, more particularly, to controlling defects during the formation of device features on the basis of advanced photolithography techniques using lithography masks.
2. Description of the Related Art
The fabrication of microstructures, such as integrated circuits, requires tiny regions of precisely controlled size to be formed in one or more material layers of an appropriate substrate, such as a silicon substrate, a silicon-on-insulator (SOI) substrate or other suitable carrier materials. These tiny regions of precisely controlled size are typically defined by patterning the material layer(s) by applying lithography, etch, implantation, deposition processes and the like, wherein, typically, at least in a certain stage of the patterning process, a mask layer may be formed over the material layer(s) to be treated to define these tiny regions. Generally, a mask layer may consist of or may be formed by means of a layer of photoresist that is patterned by a lithographic process, typically a photolithography process. During the photolithographic process, the resist may be spin-coated onto the substrate surface and then selectively exposed to radiation, typically ultraviolet radiation, through a corresponding lithography mask, such as a reticle, thereby imaging the reticle pattern into the resist layer to form a latent image therein. After developing the photoresist, depending on the type of resist, positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist. Based on this resist pattern, actual device patterns may be formed by further manufacturing processes, such as etch, implantation, anneal processes and the like. Since the dimensions of the patterns in sophisticated integrated microstructure devices are steadily decreasing, the equipment used for patterning device features have to meet very stringent requirements with regard to resolution and overlay accuracy of the involved fabrication processes. In this respect, resolution is considered as a measure for specifying the consistent ability to print minimum size images under conditions of predefined manufacturing variations. One important factor in improving the resolution is represented by the lithographic process, in which patterns contained in the photomask or reticle are optically transferred to the substrate via an optical imaging system. Therefore, great efforts are made to steadily improve optical properties of the lithographic system, such as numerical aperture, depth of focus and wavelength of the light source used.
The resolution of the optical patterning process may therefore significantly depend on the imaging capability of the equipment used, the photoresist materials for the specified exposure wavelength and the target critical dimensions of the device features to be formed in the device level under consideration. For example, gate electrodes of field effect transistors, which represent an important component of modern logic devices, may be 50 nm and even less for currently produced devices with significantly reduced dimensions for device generations that are currently under development. Similarly, the line width of metal lines provided in the plurality of wiring levels or metallization layers may also have to be adapted to the reduced feature sizes in the device layer in order to account for the increased packing density. Consequently, the actual feature dimensions may be well below the wavelength of currently used light sources provided in current lithography systems. For example, currently in critical lithography steps, an exposure wavelength of 193 nm may be used, which therefore may require complex techniques for finally obtaining resist features having dimensions well below the exposure wavelength. Thus, highly non-linear processes are typically used to obtain dimensions below the optical resolution. For example, extremely non-linear photoresist materials may be used, in which a desired photochemical reaction may be initiated on the basis of a well-defined threshold so that weakly exposed areas may not substantially change at all, while areas having exceeded the threshold may exhibit a significant variation of their chemical stability with respect to a subsequent development process.
The usage of highly non-linear imaging processes may significantly extend the capability for enhancing the resolution for available lithography tools and resist materials.
Due to the complex interaction between the imaging system, the resist material and the corresponding pattern provided on the reticle even in highly sophisticated imaging techniques, which may possibly include optical proximity corrections (OPC), phase shifting masks and the like, the consistent printing of latent images, that is, of exposed resist portions which may be reliably removed or maintained, depending on the type of resist used, may also significantly depend on the specific characteristics of the respective features to be imaged. Furthermore, the respective process parameters in such a highly critical exposure process may have to be controlled to remain within extremely tight process tolerances, which may contribute to an increasing number of non-acceptable substrates, especially as highly scaled semiconductor devices are considered. Due to the nature of the lithography process, the corresponding process output may be monitored by respective inspection techniques in order to identify non-acceptable substrates, which may then be marked for reworking, that is, for removing the exposed resist layer and preparing the respective substrates for a further lithography cycle. However, lithography processes for complex integrated circuits may represent one of the most dominant cost factors of the entire process sequence, thereby requiring a highly efficient lithography strategy to maintain the number of substrates to be reworked as low as possible. Consequently, the situation during the formation of sophisticated integrated circuits may increasingly become critical with respect to throughput.
An important aspect in reducing failures associated with advanced lithography processes may be related to the photomasks or reticles that are used for forming the latent images in the resist layer of the substrates. In modern lithography techniques, typically an exposure field may be repeatedly imaged into the resist layer, wherein the exposure field may contain one or more die areas, the image of which is represented by the specific photomask or reticle. In this context, a reticle may be understood as a photomask in which the image pattern is provided in a magnified form and is then projected onto the substrate by means of an appropriate optical projection system. Thus, the same image pattern of the reticle may be projected multiple times onto the same substrate according to a specified exposure recipe, wherein, for each exposure process, the respective exposure parameters, such as exposure dose, depth of focus and the like, may be adjusted within a predetermined process window in order to obtain a required quality of the imaging process for each of the individual exposure fields. Thus, an exposure recipe may be defined by determining an allowable range of parameter values for each of the respective parameters, which may then be adjusted prior to the actual exposure process on the basis of appropriate data, such as an exposure map, and the like. Furthermore, prior to each exposure step, an appropriate alignment procedure may be performed to precisely adjust one device layer above the other on the basis of specified process margins.
During the entire exposure process, a plurality of effects may be created, which may be associated with any deficiencies or imperfections of the exposure tool, the substrate and the like. In this case, a plurality of defects may be generated, the occurrence of which may be systematic or random and may require respective tests and monitoring strategies. For example, a systematic drift of tool parameters of the exposure tools may be determined on the basis of regular test procedures, while substrate-specific defects may be determined on the basis of well-established wafer inspection techniques to locate respective defects, such as particles and the like. Another serious source of defects may be the photomask or reticle itself, due to particles on the reticle, damaged portions and the like. As previously explained, in sophisticated lithography techniques, a plurality of measures have to be implemented in order to increase the overall resolution, wherein, for instance, in many cases, phase shift masks may be used, which comprise portions with an appropriately defined optical length to obtain a desired degree of interference with radiation emanating from other portions of the reticle. For example, at an interface between a light-blocking region and a substantially transmissive region of the mask, respective diffraction effects may result in blurred boundaries, even for highly non-linear resist materials. In this case, a certain degree of destructive interference may be introduced, for instance, by generating a certain degree of phase shift of, for instance, 180 degrees, while also providing a reduced intensity of the phase shifted fraction of the radiation, which may result in enhanced boundaries in the latent image of the resist between resist areas corresponding to actually non-transmissive and transmissive portions in the photomask. Consequently, for certain types of reticles, a change of the absorption may result in a defect in the corresponding latent image in the resist layer, which may then be repeatedly created in each exposure field. Similarly, any other defects in the reticle may result in repeated defects, which may cause a significant yield loss if the corresponding defects may remain undetected over a certain time period. There are many reasons for failures caused by reticle defects, such as insufficiency of the manufacturing sequence for forming reticles, defects created during reticle transport and reticle handling activities and the like.
One class of reticle defects has recently drawn much attention, since the detection thereof, as well as temporal development of such defects, has not been very well understood. These defects mainly occur in connection with highly sophisticated lithography techniques using short wavelengths, such as 193 nm.
FIG. 1a schematically illustrates a cross-sectional view of a typical photomask or reticle, which may be used in sophisticated lithography techniques. The reticle 100 may comprise a substrate material 101, which may be substantially transparent for a specified exposure wavelength. For instance, quartz materials may frequently be used for lithography processes using deep UV radiation, since, in this case, the intrinsic absorption in this wavelength range may still be acceptable for transmissive optical components. The substrate material 101 may have formed specific non-transparent portions 102, which represent areas that are not to be exposed so as to become soluble or remain non-soluble, depending on the type of resist material used. For instance, the portions 102 may be formed of chromium, molybdenum silicide or any other appropriate material. As previously explained, the reticle 100 may represent a phase shift mask having areas 103 creating a specific shift in phase, while also possibly attenuating the incoming radiation to enhance the resolution of the optical imaging process, as explained above. A certain degree of phase shift may be created by providing the areas 103 with a specific optical thickness with respect to substantially trans-parent portions of the substrate material 101 in order to obtain a desired phase shift between the respective light beams passing through the portion 103 and transparent portions of the substrate 101. Furthermore, the reticle 100 may comprise a transparent polymer layer 104, which may also be referred to as a pellicle, that is provided with a certain distance to the pattern defined by the substrate material 101, the areas 103 and the features 102. Pellicles have been developed to protect the sensitive surface of reticles in view of defects, such as particles and scratches caused by reticle handling and the like. On the basis of the pellicle 104, the reticle 100 may be used in production over an extended lifetime compared to non-protected reticles, which may significantly contribute to a reduction of the overall production costs, since the manufacturing of reticles may be very expensive. For example, contamination of the pellicle 104 by particles during usage of the reticle 100 may not result in respective defects in the latent image created in the resist layer since the particles on the reticles may not be within the focal plane compared to the features 102 on the surface of the substrate material 101.
During an exposure process, the reticle 100 may be exposed to incoming radiation 105 of an appropriate wavelength range, which may pass through the reticle 100, thereby creating a beam 106 including a radiation pattern corresponding to the substrate material 101, the areas 103 and the features 102. As previously explained, a great number of exposure processes may be performed on the basis of a single reticle, until a general degradation may require the replacement of the reticle 100. With the introduction of DUV lithography processes, for instance using 193 nm, a specific type of reticle defect has emerged, which may not be prevented by the provision of the pellicle 104, or which may even be enhanced by the presence of the pellicle 104.
FIG. 1b schematically illustrates the reticle 100 after a plurality of exposure processes, thereby resulting in a defect 107, which may be referred to as a crystal growth defect, which may represent a photo-induced formation of a defect area, for instance, caused by out-gassing components of the pellicle 104 or any other components, such as adhesives and the like, or by components that may be present in the ambient of the reticle 100 during exposure. The defect 107 may thus “develop” over process time of the reticle 100 and may finally reach a stage in which a significant defect may also be created in the resist layer, thereby generating a repeating defect in the exposed substrates.
For this reason, sophisticated mask inspection techniques have been developed in order to identify respective defects creating repeating defects in product substrates on a regular basis to reduce the probability of causing a significant yield loss.
FIG. 1c schematically illustrates the reticle 100 according to a typical inspection technique. For this purpose, a light source, such as a laser 110, may direct light onto the reticle 100 on the basis of an appropriate optical system (not shown), while respective detectors 111 and 112 may detect the reflected part and the transmitted part of the initial incident light beam. Based on the corresponding signals of the detectors 111, 112, appropriate defect finding algorithms may be used in order to identify relevant defects in the reticle 100. For example, whenever a new reticle is obtained, a corresponding inspection test may be performed and may be repeated on a regular basis to monitor the degradation of the reticle 100. It turns out, however, that, for sophisticated photomasks, for instance comprising phase shift areas and the like, a reliable detection of defects, such as the crystal growth defect 107, is difficult, thereby requiring an increased frequency for the testing of the available reticles while only resulting in a moderately low probability for causing a repeating defect for a moderately large number of product substrates.
The present disclosure is directed to various methods that may avoid, or at least reduce, the effects of one or more of the problems identified above.